1. Field of the Invention
The present invention relates to a method of forming a metal film.
2. Description of the Related Art
A metal film that is formed on a substrate is used for wiring or the like of various electric appliances by etching the same in a patterned manner. In a metal film formed on a substrate (metal substrate), the adhesion of the metal film to the substrate has been improved by an anchoring effect that is obtained by roughening the surface of the substrate. As a result, there has been a problem in that when the metal film is used for electric wiring, high-frequency characteristics of the same may deteriorate due to the interface portion of the metal film with the substrate being rough. Further, in order to perform surface roughening of the substrate, there is a need to conduct such a complicated step as treating the substrate with a strong acid such as chromic acid. Therefore, improvements in the above process have been demanded.
In order to address the aforementioned problems, Japanese Patent Application Laid-Open (JP-A) No. 58-196238 discloses a method of forming a metal film by applying a coating composition containing a monomer to a substrate, introducing a surface graft polymer onto the substrate by irradiating the coating composition with electron beams or UV rays, and then performing electroless plating. According to this method, a metal film can be formed without roughening the surface of the substrate. However, since this method includes a step of forming a graft polymer by applying a monomer in the form of a liquid to a substrate and irradiating the monomer in the form of a liquid with electron beams or UV rays, problems in handling in manufacturing may occur. Further, detailed investigation concerning the actual state of the substrate surface or adhesion between the substrate and the metal layer has not yet been carried out for this method.
As a method of improving adhesiveness of a metal layer with respect to a substrate by introducing a graft polymer to the substrate surface, En Tang Kang and Yan Zhang, “Advanced Materials”, 20, pp. 1481-1494 and N. Inagaki, S. Tasaka and M. Matsumoto, “Macromolecules”, 29, pp. 1642-1648 disclose a method of improving adhesiveness of a copper layer with respect to a polyimide substrate. This method includes subjecting a polyimide substrate to a plasma treatment in order to introduce a polymerization initiation group onto the surface of the polyimide surface, and then introducing a graft polymer onto the polyimide substrate by polymerizing a monomer from the polymerization initiation group, thereby forming a metal layer (copper layer) on the graft polymer. However, since this method includes such a large-scale process as a plasma treatment that consumes a great amount of energy, there has been a demand for a more convenient method.